(tau03_opt_search): Add option "ordering" (off by default).
If enabled, initialize an explicit ordering for acceptance
conditions into the new member "cond" (a vector of bdds).
(project_acc): New helper function for projecting a set of
acceptance conditions to a subset that maximizes the number
of initial consecutive conditions covered by the set in the
condition ordering.
(dfs_blue): Implement the ordering heuristic.
(dfs_red): Use a sentinel in condition_stack to avoid explicit
checks for the stack's emptiness.
Consider also the conditions in acc when checking for the
completion of an accepting cycle.
Fix the implementation of condition heuristic.
Implement the ordering heuristic.
Simplify the removal of elements from condition_stack (due to
the way in which elements are pushed on the stack, there can
be at most one element with a given depth in the stack at any
one time).